
Pilots doors.
Three things come to mind.
1- It was usually 90' to 100'F degrees and very humid. Like sitting in the back, where I was, The breeze felt good.
2- With the weapon fire going on, the cockpit would fill with smoke. No doors or open doors helped clear out the work space.
3- When the going got tough and you were going to have to walk home, once on the ground, you wanted out.
A hard ( hopefully that was all it was ) landing could distort the fuselage ( body ) and if you had doors shut, they might jam. You could not get out. This was real bad. So, just before ground contact, you either opened the doors or took them off to start with.